Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 7 Drill 1
Using std_lib_facilities.h by Bjarne Stroustrup.
Drill 1. Starting from the file calculator08buggy.cpp, get the calculator to compile.
[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP
// Chapter 7 Drill 1
/*
Get the calculator to compile.
calculator08buggy.cpp
Helpful comments removed.
We have inserted 3 bugs that the compiler will catch and 3 that it won’t.
*/
#include "std_lib_facilities.h"
struct Token {
char kind;
double value;
string name;
Token(char ch) :kind(ch), value(0) { }
Token(char ch, double val) :kind(ch), value(val) { }
Token(char ch, string n) :kind(ch), name(n) { }
};
class Token_stream {
bool full;
Token buffer;
public:
Token_stream() :full(0), buffer(0) { }
Token get();
void unget(Token t) { buffer = t; full = true; }
void ignore(char);
};
const char let = ‘L’;
const char quit = ‘Q’;
const char print = ‘;’;
const char number = ‘8’;
const char name = ‘a’;
Token Token_stream::get()
{
if (full) { full = false; return buffer; }
char ch;
cin >> ch;
switch (ch) {
case ‘(‘:
case ‘)’:
case ‘+’:
case ‘-‘:
case ‘*’:
case ‘/’:
case ‘%’:
case ‘;’:
case ‘=’:
return Token(ch);
case ‘.’:
case ‘0’:
case ‘1’:
case ‘2’:
case ‘3’:
case ‘4’:
case ‘5’:
case ‘6’:
case ‘7’:
case ‘8’:
case ‘9’:
{ cin.unget();
double val;
cin >> val;
return Token(number, val);
}
default:
if (isalpha(ch)) {
string s;
s += ch;
while (cin.get(ch) && (isalpha(ch) || isdigit(ch))) s = ch;
cin.unget();
if (s == "let") return Token(let);
if (s == "quit") return Token(name);
return Token(name, s);
}
error("Bad token");
}
}
void Token_stream::ignore(char c)
{
if (full && c == buffer.kind) {
full = false;
return;
}
full = false;
char ch;
while (cin >> ch)
if (ch == c) return;
}
struct Variable {
string name;
double value;
Variable(string n, double v) :name(n), value(v) { }
};
vector<Variable> names;
double get_value(string s)
{
for (int i = 0; i<names.size(); ++i)
if (names[i].name == s) return names[i].value;
error("get: undefined name ", s);
}
void set_value(string s, double d)
{
for (int i = 0; i <= names.size(); ++i)
if (names[i].name == s) {
names[i].value = d;
return;
}
error("set: undefined name ", s);
}
bool is_declared(string s)
{
for (int i = 0; i<names.size(); ++i)
if (names[i].name == s) return true;
return false;
}
Token_stream ts;
double expression();
double primary()
{
Token t = ts.get();
switch (t.kind) {
case ‘(‘:
{ double d = expression();
t = ts.get();
if (t.kind != ‘)’) error("'(‘ expected");
}
case ‘-‘:
return -primary();
case number:
return t.value;
case name:
return get_value(t.name);
default:
error("primary expected");
}
}
double term()
{
double left = primary();
while (true) {
Token t = ts.get();
switch (t.kind) {
case ‘*’:
left *= primary();
break;
case ‘/’:
{ double d = primary();
if (d == 0) error("divide by zero");
left /= d;
break;
}
default:
ts.unget(t);
return left;
}
}
}
double expression()
{
double left = term();
while (true) {
Token t = ts.get();
switch (t.kind) {
case ‘+’:
left += term();
break;
case ‘-‘:
left -= term();
break;
default:
ts.unget(t);
return left;
}
}
}
double declaration()
{
Token t = ts.get();
if (t.kind != ‘a’) error("name expected in declaration");
string name = t.name;
if (is_declared(name)) error(name, " declared twice");
Token t2 = ts.get();
if (t2.kind != ‘=’) error("= missing in declaration of ", name);
double d = expression();
names.push_back(Variable(name, d));
return d;
}
double statement()
{
Token t = ts.get();
switch (t.kind) {
case let:
return declaration();
default:
ts.unget(t);
return expression();
}
}
void clean_up_mess()
{
ts.ignore(print);
}
const string prompt = "> ";
const string result = "= ";
void calculate()
{
while (true) try {
cout << prompt;
Token t = ts.get();
while (t.kind == print) t = ts.get();
if (t.kind == quit) return;
ts.unget(t);
cout << result << statement() << endl;
}
catch (runtime_error& e) {
cerr << e.what() << endl;
clean_up_mess();
}
}
int main()
try {
calculate();
return 0;
}
catch (exception& e) {
cerr << "exception: " << e.what() << endl;
char c;
while (cin >> c&& c != ‘;’);
return 1;
}
catch (…) {
cerr << "exception\n";
char c;
while (cin >> c && c != ‘;’);
return 2;
}
[/code]
Output: > 6+6= = 12 > primary expected
